If it pertains to grammar, it is probably a shift in the verb tense. You might have shifted from past to present, etc. You should try to keep your tenses the same thoughtout your paper or at least within the same paragraph. If it is not referring to tense, you will need to give more information.
Is this shift used as a noun? There are many possible meanings, here are some:
1. A change from one person or configuration to another; a substitution.
a. A group of workers that relieve another on a regular schedule.
b. The working period of such a group: worked the night shift.
a. A means to an end; an expedient.
b. A stratagem; a trick.
4. A change in direction: a shift in the wind.
5. A change in attitude, judgment, or emphasis.
6. A change in position, as:
a. Music A change of the hand position in playing the violin or a similar instrument.
b. Football A rearrangement of players from one formation to another just prior to the snap of the ball.
c. Baseball A rearrangement of one or more fielders for improved defense against a particular hitter.
d. Geology See fault.
e. Computer Science Movement of characters in a register to the left or right, as of the bits in a byte.
7. An act or instance of using a shift key.
8. Physics A change in wavelength, causing a movement of a spectral band or line.
a. A systematic change of the phonetic or phonemic structure of a language.
b. Functional shift.
a. A loosely fitting dress that hangs straight from the shoulder; a chemise.
b. A woman's undergarment; a slip or chemise.